The present invention relates to intramedullary nails used for treatment of a fracture of a bone having a medullary canal extending longitudinally within the bone, and particularly to the structure of the intramedullary nail and methods for anchoring the intramedullary nail with respect to one or more fragments of the fractured bone. Intramedullary nails are used by orthopedic surgeons to treat fractures involving long bones such as the femur, humerus, tibia, fibula, etc. The medullary canal of the fractured bone is drilled out or otherwise opened from one end, and the intramedullary nail is longitudinally placed within the medullary canal to contact at least two fragments, i.e., such that the nail extends on both sides of the fracture. As used herein, the term xe2x80x9cfragmentxe2x80x9d refers to a portion of a fractured bone regardless of whether the fracture is complete. When implanted, the nail strengthens and supports fragments of the fractured bone during healing of the fracture.
Various types of intramedullary nails are well known within the medical device arts, and several different methods have been used to attach the intramedullary nail within the bone. For instance, in U.S. Pat. No. 4,338,926 to Kummer et al., an intramedullary nail is disclosed which places a compressive force radially outward on the interior wall of the cortex structure surrounding the intramedullary nail. The compressive force secures the Kummer nail within the medullary canal of the fragments. Similarly, in U.S. Pat. No. 4,457,301 to Walker a flexible plastic core elements holds longitudinal pins of an intramedullary nail in place. In U.S. Pat. No. 5,514,137 to Coutts, cement is injected through a cannula in an intramedullary nail to secure the distal end of the intramedullary nail to the bone. Other intramedullary nail designs employ a more secure and mechanically positive attachment to the bone, such as through use of one or more bone fasteners which extend transversely to the longitudinal axis of the nail and through the cortex of the bone. The bone fastener is received within a receiving recess or through-hole within the intramedullary nail to secure the intramedullary nail relative to the bone fastener. In the transverse attachment, the receiving opening defines an axis which is at an angle to the longitudinal axis of the nail (90xc2x0 and 45xc2x0 angles are common), and the bone fastener is advanced on this receiving opening axis. U.S. Pat. No. 4,733,654 to Marino, U.S. Pat. No. 5,057,110 to Kranz et al., U.S. Pat. No. 5,127,913 to Thomas, Jr., U.S. Pat. No. 5,514,137 to Coutts (proximal end) and others disclose such a transverse bone fastener attachment in a bicortical attachment. U.S. Pat. No. 5,484,438 to Pennig shows a nail design with a recess which permits only unicortical attachment. The present invention particularly relates to intramedullary nails which use bone fasteners transversely through the cortex for attachment.
Problems may arise when attaching an intramedullary nail to a fragment with a bone fastener. It is occasionally difficult for the surgeon to properly align the bone fastener and/or a hole for the bone fastener with the receiving opening on the nail. Part of the error is simply due to difficulty in aligning the bone fastener with the receiving opening when the receiving opening is within the bone. Additionally, the nail may be slightly bent during insertion of the nail structure into the medullary canal. Such bending of the nail structure may be desired in some instances so the nail shape better matches the particular shape of the medullary canal for a particular patient. Regardless of whether intended or unintended, bending of the nail structure creates further alignment errors between the bone fastener and/or a hole for the bone fastener and the receiving opening on the nail. Four types of alignment errors can be identified: (a) in transverse displacement (e.g., when the axis of the bone fastener is in the same transverse plane as the receiving opening in the nail but does not intersect the axis of the nail), (b) in longitudinal displacement (i.e., when the bone fastener is at a different longitudinal location than the receiving opening in the nail), (c) in longitudinal angular misalignment (i.e., when the axis of the receiving opening and the axis of the bone fastener are at different angles relative to the longitudinal axis of the nail), and (d) in transverse angular misalignment (i.e., when the axis of the receiving opening and the axis of the bone fastener are in the same transverse plane but at different radial positions relative to the nail).
Various types of jigs have been proposed to reduce alignment errors, such as shown in U.S. Pat. No. 4,733,654 to Marino and U.S. Pat. No. 5,776,194 to Mikol et al. The jig may be temporarily attached to the proximal end of the nail to help align the bone fastener and/or the drill to the receiving opening in the nail. While such jigs are helpful, they become less reliable as distance from the proximal end of the nail increases, particularly if any bending of the intramedullary nail has occurred. Additional solutions are needed, especially for attaching the distal end of the intramedullary nail to a distal fragment.
A second method to reduce such alignment problems is to locate the receiving openings in-situ, such as through an x-ray or through the use of magnets as taught in U.S. Pat. No. 5,127,913 to Thomas, Jr. Such methods are not typically preferred by surgeons in as much as they require significant additional time and effort during the orthopedic surgery, to the detriment of the patient.
A third method to reduce such alignment problems is to drill the receiving opening into the intramedullary nail only after the nail is placed into the bone, allowing the receiving opening to be formed at a range of locations. Such in-situ drilling is taught in U.S. Pat. No. 5,057,110 to Kranz et al., wherein a tip section of the intramedullary nail is formed of a bioresorbable material. However, bioresorbable materials are not as strong as metals, leading to a product which is weaker than desired and has a weaker attachment than desired.
Further problems with intramedullary nails occur during placement of the intramedullary nail. For minimal damage to cortical tissue of the bone and most beneficial healing, both the hole that is drilled in the medullary canal for the nail and then the nail itself need to be precisely located and secured with respect to the medullary canal.
Additional problems with intramedullary nails occur due to the healing requirements of the bone with respect to the strength and rigidity of the nail. U.S. Pat. No. 4,756,307 to Crowninshield and U.S. Pat. No. 4,338,926 to Kummer et al. disclose intramedullary nails with bioresorbable portions to weaken the nail relative to the bone over time, but these nails forsake the use of a transverse bone fastener to achieve this benefit.
The present invention involves an intramedullary nail for treatment of a fracture of a bone by placement of the intramedullary nail within the medullary canal of the fractured bone. The nail structure is formed with at least one window in an exterior side, and a spacer of a non-metal material is positioned within the window. In one aspect of the invention, the non-metal spacer is formed of a bioresorbable material, and the window is a dynamization window. The nail is used with a bone fastener such as a bone screw which is advanced transversely through the bone and into the spacer, preferably in a bicortical attachment with the bone. The bone fastener is smaller across than the window, so the spacer spaces the bone fastener relative to the metal structure of the nail. The window may have a longitudinal length that is different from its width, while the bone fastener has a circular cross-section. Because the bone fastener is smaller across than the window and spacer, a larger error in placement of the bone fastener is permissible. Also, as the bioresorbable spacer resorbs, stress is increasingly transmitted through the fracture site rather than through the intramedullary nail. The positioning of the bone fastener, the shape and size of the window and spacer, and the material of the spacer all allow design control over the type and amount of dynamization seen at the fracture site. Use of a separate spacer, which is placed into the nail structure at the time of implantation by the surgeon, allows the surgeon to select a non-metal spacer which has appropriate features and/or had been appropriately treated and handled to best match the desired healing modality of the particular fracture.